The present technique relates generally to the field of electric motors and to methods and apparatus for detecting temperatures within an electric motor. More particularly, the technique relates to a novel approach to detecting the temperature adjacent to a winding within an electric motor.
Electric motors, generators, and other similar devices, are produced in a variety of mechanical and electrical configurations. The configuration of these devices may depend upon the intended application, the operating environment, the available power source, or other similar factors. In general, these devices include a rotor surrounded at least partially by a stator. For instance, one common design of electrical motor is the induction motor, which is used in numerous and diverse applications. Induction motors typically employ a stator assembly including a slotted core in which groups of coil windings are installed. By providing alternating current power to certain windings at certain times, a dynamic magnetic field is produced that causes the rotor to rotate within the stator. The rotational speed of the rotor is a function of the frequency of the alternating current power input and of the motor design (i.e. the number of poles defined by the windings). This rotation may be used to transmit a mechanical force to a driven load via an output shaft coupled between the rotor and the driven load.
Inverter drives are commonly used with such motors to vary the frequency of the alternating current power driving the motor. This, accordingly, allows the rotational speed of the rotor to be varied as well. An inverter drive is configured to receive a direct current input and to output a variable frequency waveform that simulates alternating current power. A rectifying circuit may be used with the inverter drive if alternating current power is being provided to the inverter drive. In such a case, the rectifier converts the incoming alternating current power to direct current power for input into the inverter circuitry.
Electric motors and other similar devices are generally configured to operate in a given temperature range. Heat is generated within the motor from the passage of electrical current through the coil windings, or from a variety of other sources. Often, one or more resistance temperature detectors will be disposed within one or more slots of a stator core, adjacent to the coil windings. In this manner, the temperature of the coil windings can be measured to determine if the temperature is within desired operating parameters. However, these resistance temperature detectors must remain operable in order to provide this benefit.
In general, a resistance temperature detector includes a resistance coil encased in an insulative cover. Current is applied to leads of the coil that results in a measurable voltage drop through the coil as a function of the resistance of the coil. Because the resistance varies with the temperature of the coil, by measuring the voltage drop, compensating for certain errors, and correlating the voltage drop to the known characteristics of the coil, the temperature at the detector location can be calculated. In motor applications this temperature signal may then be used for various monitoring, control, preventative maintenance, and other functions.
The environments in which conventional resistance temperature detectors are placed are often severe, particularly in high power motors. The detectors are subjected to elevated temperature levels and, at times to significant voltage differentials, particularly in inverter driven applications. The corresponding voltage stress and electric field within these environments may lead to premature failure of resistance temperature detectors. Particularly, the electric field within the motor may electrically break down small air gaps or other materials or zones of reduced dielectric constant within a resistance temperature detector, causing partial discharge and localized heating within such zones. If any air voids or low dielectric materials are disposed adjacent to the resistive element of the detector, this localized heating may cause the detector to report inaccurate temperatures or may cause the resistive element to deteriorate, resulting in the failure of the detector. Moreover, winding insulation systems are being further reduced in thickness, leading to increasing voltage stresses, particularly across temperature detector insulating layers. Such elevated stresses can similarly lead to failure of the detectors.
There is, therefore, a need for a resistance temperature detector that can better withstand the harsh electrical environment present in many applications, and particularly within a slot of a stator core. Such a device would allow for the monitoring of temperatures of windings of electric motors or similar devices, and thereby prevent damage and downtime, and improve reliability of the entire motor system.